OAKLAND — Closing arguments were delivered Thursday in the trial of Quochuy “Tony” Tran, who is accused of fatally shooting Ichinkhorloo “Iko” Bayarsaikhan, 15, at an Alameda park on Halloween three years ago.

“In many ways, this is as simple and straightforward a case you can see as a jury,” deputy district attorney Tim Wellman said in his remarks. “Let me be very clear about this: The defendant is the killer.”

Tran is accused of firing the shot that killed Bayarsaikhan after entering Washington Park on Oct. 31, 2007. He had come to the park with five other boys, who approached Bayarsaikhan’s group of friends with the intention of robbing them.

After demanding money, one of the boys fired four shots into the air with a .22-caliber rifle. Boys from Bayarsaikhan’s group began to walk toward Tran’s friends, at which point Tran took the gun from the other boy, authorities have said.

According to Wellman, Tran told the other group, “This ain’t no cap gun. You think I’m playing?”

He then fired one shot, hitting Bayarsaikhan in the chest, Wellman said. She was pronounced dead at Highland Hospital in Oakland at 10:51 p.m. that night.

Defense attorney Anne Beles contested several aspects of the prosecution’s case. The boy who had fired four shots into the air during the robbery attempt later told jurors that he also had fired the final shot, which killed Bayarsaikhan. This contradicted previous statements he had given to police, as well as the testimony of other individuals who had been at the park that night.

Five boys, including the one who later claimed to have fired the final shot, have been convicted in juvenile court for their roles in the slaying. Tran is being tried as an adult and has been charged with murder with enhancement for personal use of a weapon. He faces life in prison if convicted.

“The fact is, the prosecution has not met the burden of proof,” Beles said. “We have very conflicting evidence as to who went up to them with the gun.”

She added that the “switch-a-roo” of possession of the gun made it difficult to ascertain who fired when.

“No matter how many times the prosecution says, ‘Everyone there was found guilty,’ it doesn’t mean Tony Tran is guilty,” she said.

According to Wellman, even if Tran did not fire the gun, he was still a willing participant in an attempted armed robbery that resulted in a murder. Beles said the attempted robbery had been aborted by the time the fatal shot had been fired.

Members of Bayarsaikhan’s family attended Thursday’s proceedings. One woman wept as Wellman showed the jury the last known photograph of Bayarsaikhan, taken from a McDonald’s security camera on the night of her death.

“He could’ve chosen to do so many things, but he chose to kill Iko. He chose to pull the trigger,” he said.

Tran appeared calm and sat with his elbows on the table, occasionally picking up a pen and marking a notepad.

Bayarsaikhan moved to Alameda with her brother and parents six years ago. At the time of her death, she was a junior at Alameda High School.

Tran, who was 16 at the time of the slaying, was a student at Oakland High School. According to Wellman, at least one boy was heard yelling “Oakland High” as he ran from Washington Park.

In her closing remarks, Beles said the confusion surrounding the firing of the gun, as well as the aborted attempted robbery, established enough doubts for the jury to return a not-guilty verdict.

“Probably guilty doesn’t do it. Might be guilty doesn’t do it. Only beyond a reasonable doubt does it,” she said. “And do you have reasons to doubt? Yes.”

Tran’s trial began May 17, with opening statements June 3. The jury will begin deliberating at 9:30 a.m. Monday.